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1.
G.R. Bigg 《Dynamics of Atmospheres and Oceans》1984,8(1):87-94
The formation of two regions of vorticity, one cyclonic and one anticyclonic, after the initiation of flow is discussed. In particular, the linearized case of elongated topography is examined. The non-linear interaction of the two vortices, once formed, is discussed using a simple model. 相似文献
2.
The possible future impact of anthropogenic forcing upon the circulation of the Mediterranean, and the exchange through the
Strait of Gibraltar is investigated using a Cox-type model of the Mediterranean at 0.25° × 0.25° resolution, forced by “control”
and “greenhouse” scenarios provided by the HadCM2 coupled climate model. The current structure of the Mediterranean forced
by the “control” climate is compared with observations: certain aspects of the present circulation are reproduced, but others
are absent or incorrectly represented. Deficiencies are most probably due to weaknesses in the forcing climatology generated
by the climate model, so some caution must be exercised in interpreting the enhanced greenhouse simulation. Comparison of
the control and greenhouse scenarios suggests that deep-water production in the Mediterranean may be reduced or cease in the
relatively near future. The results also suggest that the Mediterranean outflow, may become warmer and more saline, but less
dense, and hence shallower. The volume of the exchange at the Strait of Gibraltar seems to be relatively insensitive to future
climate change, however. Our results indicate that a parameterisation of Gibraltar exchange and Mediterranean Outflow Water
(MOW) production may be able to provide adequate representation of the changes we observe for the purposes of the current
generation of climate models.
Received: 10 August 1998 / Accepted: 11 October 1999 相似文献
3.
David J. Wilton Grant R. Bigg James D. Scourse Jeremy C. Ely Chris D. Clark 《第四纪科学杂志》2021,36(5):934-945
The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice-rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment-level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000-year time steps during the height of the last BIIS glaciation (31–6 ka bp ). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances. 相似文献
4.
The precipitation fields of a palaeoatmospheric general circulation model are used to derive estimates of the geographical distribution, and flux, of icebergs from the Laurentide, Fennoscandinavian and eastern Siberian ice‐sheets at the Last Glacial Maximum (LGM). The atmospheric model fields from LGM simulations using CLIMAP or Peltier (ICE‐4G) ice orography were studied, to test the sensitivity of the predicted flux. The estimated Northern Hemispheric LGM iceberg flux is 3500–4000 km3 yr?1, of which about 60% issued directly into the North Atlantic. The iceberg flux from the St Lawrence area is of similar significance to that issuing from Hudson Strait in all estimates. Both the North Pacific and the Arctic received substantial iceberg fluxes (ca. 700 km3 yr?1), with relatively minor differences occurring between the two ice‐sheet reconstructions. Apparent discrepancies between Arctic deep‐sea core samples of ice‐rafted debris and our estimates of mean glacial iceberg flux may be ascribed to coastal trapping of bergs, the existence of floating ice tongues or a rapid exit of icebergs from the Arctic basin into the Greenland Sea through the Fram Strait. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
5.
Past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the Atlantic Meridional Overturning Circulation and consequently climate. However, very little is known about past changes in the vertical density structure of the ocean, even very extensively studied systems such as the North Atlantic. Here we exploit the physical controls on the settling depth of the dense Mediterranean water plume derived from the Strait of Gibraltar to obtain the first robust, observations-based, probabilistic reconstruction of the vertical density gradient in the eastern North Atlantic during the last 30,000?years. We find that this gradient was weakened by more than 50%, relative to the present, during the last Glacial Maximum, and that changes in general are associated with reductions in AMOC intensity. However, we find only a small change during Heinrich Event 1 relative to the Last Glacial Maximum, despite strong evidence that overturning was substantially altered. This implies that millennial-scale changes may not be reflected in vertical density structure of the ocean, which may be limited to responses on an ocean-overturning timescale or longer. Regardless, our novel reconstruction of Atlantic density structure can be used as the basis for a dynamical measure for validation of model-based AMOC reconstructions. In addition, our general approach is transferrable to other marginal sea outflow plumes, to provide estimates of oceanic vertical density gradients in other locations. 相似文献
6.
British summers are known to have become warmer and drier in recent decades, but has there also been a change in the timing of this season’s beginning, as has occurred for spring? This paper addresses this question by examining records of the first blooming date of early summer flowering plants (phenology) and the timing of first occurrences of warm ‘summer’ temperatures, both events that we associate with the onset of summer. This has revealed that the onset of summer in England has been advancing since the mid 1950s. The occurrence of ‘summer’ temperatures has advanced more than early summer flowering, by eleven and three days respectively in the 1990s compared to the period 1954–1963. This may have encouraged drought or heat wave conditions by prolonging the period of warm temperatures and lower rainfall in which these events occur. 相似文献
7.
A global fine resolution curvilinear ocean model, forced by NCEP Re-Analysis fluxes, is used to study changes in the circulation of the Nordic Seas and surrounding ocean basins during 1994-2001. The model fields exhibit regionally distinct temporal variability, mostly determined by atmospheric forcing but in regions of significant sea-ice longer timescale variability is found. Some abrupt circulation changes accompany the relaxation of the westerlies following the peak North Atlantic Oscillation Index phase of the mid 1990s. The Greenland gyre spins up over the following years, with the increased circulation partially exiting through the Denmark Strait into the northern Atlantic as well as re-circulating within the Nordic Seas. This resulted in a distinct freshening around northern Iceland and an increase in the East Icelandic Current. However, these latter increases steadied after 1998, as the increased Greenland Sea gyre circulation led to a greater proportion of water leaving through the Denmark Strait, rather than re-circulating. The model Denmark Strait Outflow therefore doubles during the latter half of the 1990s. Increased convection in the Icelandic Sea in the model in 1998-2001 acted to obliterate the anomalies that would otherwise have fed into the East Icelandic Current. A fresh, cold anomaly from the Arctic during 1998/1999 is shown to propagate through the system. Model and observations show good agreement generally, but diverge at depth more in the last few years of the simulation. The model shows that density anomalies within the East Greenland Current do not exclusively derive from the Arctic but may also arise from air-sea interaction within the Greenland Sea. Convection is a major means of limiting anomaly propagation within the model. The contrast of climatological with daily forcing shows the inherent strength of the variability in the ocean circulation on sub-decadal timescales. 相似文献
8.
Seasonal variations in total particle concentrations at five remote Southern Hemisphere sites in Samoa, Tasmania and Antarctica are compared. The dominant particle production mechanism is deduced to be photochemical, with concentrations dictated by the availability of radiation in the eight months centred on the winter solstice. There is little variation with latitude in the mean summer concentrations. The middle troposphere is shown to act as an important reservoir of photochemically-produced particles in Tasmania and probably in the Antarctic. It is suggested that primary-particle production is mainly through the oxidation of dimethyl sulphide and that these particles then act as centres of condensation for sulphuric acid formed by the oxidation of sulphur dioxide. 相似文献
9.
Controversy exists over the extent of glaciation in Eastern Asia at the Last Glacial Maximum: complete ice sheet cover vs. restricted mountain icefields (an area discrepancy equivalent to 3.7 Greenland Ice Sheets). Current arguments favour the latter. However, significant last glacial ice-rafted debris (IRD) exists in NW Pacific ocean cores, which must have been sourced from a major ice sheet somewhere bordering the North Pacific. The origin of this IRD is addressed through a combination of marine core analysis, iceberg trajectory modelling and remote sensing of glacial geomorphology. We find compelling evidence for two stages of glaciation centred on the Kamchatka area of maritime southeast Russia during the last glacial, with ice extent intermediate in size between previous maximum and minimum reconstructions. Furthermore, a significant increase in iceberg flux precedes, and accompanies, a substantial marine core ash deposit at around 40 ka BP. We speculate that rapid decay of the first stage of the ice sheet may have triggered substantial volcanic activity. 相似文献
10.
The Atlantic meridional overturning circulation (AMOC), an important component of the climate system, has only been directly measured since the RAPID array’s installation across the Atlantic at 26°N in 2004. This has shown that the AMOC strength is highly variable on monthly timescales; however, after an abrupt, short-lived, halving of the strength of the AMOC early in 2010, its mean has remained?~?15% below its pre-2010 level. To attempt to understand the reasons for this variability, we use a control systems identification approach to model the AMOC, with the RAPID data of 2004–2017 providing a trial and test data set. After testing to find the environmental variables, and systems model, that allow us to best match the RAPID observations, we reconstruct AMOC variation back to 1980. Our reconstruction suggests that there is inter-decadal variability in the strength of the AMOC, with periods of both weaker flow than recently, and flow strengths similar to the late 2000s, since 1980. Recent signs of weakening may therefore not reflect the beginning of a sustained decline. It is also shown that there may be predictive power for AMOC variability of around 6 months, as ocean density contrasts between the source and sink regions for the North Atlantic Drift, with lags up to 6 months, are found to be important components of the systems model. 相似文献